Discontinuous electrode for electrophotography

ABSTRACT

A persistent internal polarization (PIP) electrophotography printing system comprising at least one electrode of a discontinuous configuration, such as a foraminous conductive mesh, in combination with a PIP layer such that an electric field may be simultaneously applied to the PIP layer while permitting light radiation to reach the PIP layer. In the specific practice, the electrode is positioned adjacent to the surface of the PIP layer which is to be toned; that is, the top surface.

United States Patent Hall Mar. 14, 1972 [54] DISCONTINUOUS ELECTRODE FOR 2,825,814 3/1958 Walkup ..317/262 A UX ELECTROPHOTOGRAPHY 2,833,930 5/1958 Walkup.... ....317/262 A X Inventor: Maclin S Hall Okemos, Mich. 3,268,331 8/1966 Harper ..355/l7 X [73] Assignee: Owens-Illinois, lnc. Primary Examiner-Samuel S. Matthews Assistant Examiner-Kenneth C. Hutchison [22] Ffled' July 1968 Attorney-Alan J. Steger and W. A. Schaich [21] Appl. No.: 746,192

[57] ABSTRACT [52] US. Cl. ..355/12, 317/262 A A persistent i l polarization (P1P) electrophotography 1 8 15/02 printing system comprising at least one electrode of a discon- [58] Flew of Search "355/12; 317/262 262 A, 262 AB tinuous configuration, such as a foraminous conductive mesh, 56 R i in combination with a PI! layer such that an electric field may I 1 e and be simultaneously applied to the PIP layer while permitting UNITED STATES PATENTS light radiation to reach the PIP layer. In the specific practice, the electrode is positioned adjacent to the surface of the PIP 3,005,707 10/1961 Kallmann et al ..96/l layer hi h i t be toned; that is, the top surface. 3,168,022 2/1965 Limberger ...355/12 X 3,335,003 8/ 1967 Snelling ..355/ 12 X 2 Claims, 3 Drawing Figures l2 IO [4- Patented March 14, 1972 III:

FIG

3 mm F mgmuu. NAcKuu HALL BY 2' A DISCONTINUOUS ELECTRODE FOR ELECTROPIIOTOGRAPHY BACKGROUND OF THE INVENTION This invention relates to novel apparatus and process for practicing electrophotographic printing or copying. More particularly, this invention relates to printing apparatus and process utilizing photoconductive insulating materials and the principles of persistent internal polarization.

Persistent internal polarization (abbreviated herein as PIP) involves the separation of positive and negative charges in a photoconductive insulating material by subjecting it to irradiation and an electric field. The charges are subsequently trapped and remain fixed or frozen so as to form an internal polarization field for a period of time sufficient to permit toning. PIP and the theory thereof are well known in the electrophotography art. See, for example, Electrophotography, by R. M. Schaffert, The Focal Press, London and New York (I965), pages 59 through 77, and Persistent Internal Polarization, by Kallmann and Rosenberg, The Physical Review, Volume 97, No. 5 (Mar. 15, 1955), pages I596 through 1610, both of which are incorporated herein by reference.

In general, a PIP electrophotography system includes a layer of photoconductive insulating material sandwiched between a pair of field producing electrodes. The phenomenon of PIP can be achieved in any material which exhibits the following characteristics:

1. The material must have a high resistivity in the dark (a low density of free charge carriers), whereby it is a good insulator in the absence of irradiation.

2. The material must be photoconductive. In other words, it must have decreased resistivity when excited with appropriate irradiation.

Thus, a PIP material is one which will become persistently internally polarized due to the separation of positive and negative charges when it is subjected to irradiation and the action of an electric field.

Typical PIP materials contemplated herein comprise binder dispersions of photoconductors and binder free thin films of photoconductors.

Examples of inorganic photoconductors contemplated in the process of this invention include, not by way of limitation, appropriately activated zinc sulfide, cadmium sulfide, zinc selenide, cadmium selenide, cadmium oxide, zinc-cadmium selenides, and zinc-cadmium sulfides. Examples of organic photoconductors include anthracene, chrysene, and poly (vinylcarbazole).

Examples of resin binders contemplated herein include not by way of limitation, cellulose acetate, cellulose ether, cellulose ester, silicones, vinyl resins, alkyds, and/or epoxy resins.

When it is desired to form a latent electrostatic image in the PIP material, it is flooded with radiation and an electric field is applied so as to polarize the PIP layer. After termination of the flooding radiation, the polarity of the electric field across the PIP material is reversed and the PIP materials exposed to an image or other pattern of activating radiation. The reversal of the electric field will cause rapid depolarization of that portion (s) of the PIP material rendered photoconductive under the influence of the image-wise radiation.

If the exposure to the image is continued for a sufficient time period, the irradiated area of the PIP layer will repolarize and assume a polarization opposite to that of the non-irradiated or dark portion of the PIP layer. Thus, the image is simulated by an internal latent electrostatic image or pattern detectable at the surface of the PIP material.

This latent electrostatic image is subsequently developed with charged or dipolar toner particles so as to produce a visible reproduction of the image which is capable of being viewed, photographed, or transferred, utilizing known methods in the electrophotography printing or copying art.

It should be noted that, due to the characteristics of the PIP material, the latent electrostatic image produced in the PIP material will typically remain fixed such that a finite number of reproductions can be made. The image can be erased by overall irradiation with or without an electric field, thereby returning the PIP material to a prepolarized or neutral condition capable of being used for the formation of a new electrostatic image.

The irradiation of the PIP material (for polarization and/or imaging) can be accomplished by means of any form of electromagnetic or particulate radiation or energy, visible or invisible, which will excite the PIP material so as to permit charge separation in an electric field. Such radiation includes not by way of limitation visible light, infrared, ultraviolet, X- rays gamma rays, and beta rays. For printing or copying purposes, the typical radiation is light in the visible range.

In the prior electrophotographic printing and copying art, simultaneous application of the electric field and the light from an image to a PIP material has been obtained by means of at least one continuous electrode which is substantially transparent. Typically, these electrodes have been conductive liquids or glass with conductive coatings. Conductive liquid electrodes are different to handle and the use of conductively coated glass makes it difficult to obtain uniform electrode contact. Likewise, such prior art electrode systems are limited by the spectral absorption of the substrate and/or conductive coating.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a new and novel electrode which overcomes the aforementioned disadvantages associated with the prior art devices. More particularly, this invention provides a discontinuous electrode which may be in the form of a conductive (e.g., metal or metal coated) mesh which is extremely easy to handle and conforms readily to the surface of the PIP material.

In accordance with a specific embodiment of this invention, a removable discontinuous electrode is positioned adjacent to the top surface of the layer of PIP material such that the electrode can be removed from the surface for toning and transfer.

In a specific practice hereof, this invention features the placement of a thin mesh grid electrode on the top surface of a PIP layer and a second electrode on the bottom of the layer. Typically, the second electrode is a permanent, non-removable electrode which is sufficient to support the PIP material. Although the second electrode may be discontinuous or con tinuous, the latter is usually preferred.

In operation, an electric field is applied between the base electrode and the screen mesh electrode and the radiation is impinged upon the PIP layer through the interstices of the mesh. The use of the top mesh electrode directly overcomes the spectral absorption limitations of the coated glass electrodes by providing sufficient electric field and light intensity in the PIP material so as to generate adequate PIP for printing purposes.

Other features and advantages of the subject invention will become obvious to those skilled in the art upon reference to the following detailed description and the drawings illustrating a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a schematic view of a PIP system having a removable discontinuous electrode. 1

FIG. 2 is a schematic view of the PIP system of FIG. 1 upon the formation of a latent electrostatic image.

FIG. 3 is a schematic view of a discontinuous electrode in the form of a flat screen mesh.

DESCRIPTION OF A PREFERRED EMBODIMENT In the drawings the numeral 10 refers to a body or layer of PIP material as previously defined. The PIP body 10 is sandwiched between a pair of electrodes 12 and 14 which are connected to a DC source E. For the purposes of explanation, the electrode I2 is connected to the positive terminal of the DC source E and, accordingly, the electrode 14 is connected to the negative terminal of the DC source E.

Since the electrode 14 is located between the sources of light and the PIP body 10, this electrode 14 is of a discontinuous structure, e.g., foraminous, so as to allow the impingement ofthe light into the PIP body 10. Thus, as provided by this invention, the electrode 14 is constructed of a discontinuous conductive material and may take the form of a screen mesh or like structure such as is shown in FIG. 3. One possible such mesh material would be an electroformed nickel mesh which can be obtained commercially as fine as 2,000 lines per inch. The light transmission property of the mesh electrode can be varied by controlling the space and the metal dimensions. Typical transmission in the rang of 60 to 90 percent is readily available. Thus, it can be seen that light can easily pass through the openings of such a mesh so that the electrode formed from such a mesh provides no spectral limitations on the light one may choose for imaging so as to be virtually transparent for PIP purposes.

To initially polarize the PIP body 10, the system is flooded with light as shown in FIG. 1. Under the combined action of the light and the DC source E, it is shown schematic schematically (in FIG. 1) that negative charges are effectively conducted toward the edge of the PIP body 10, which is adjacent to the electrode 12 connected to the positive terminal of the DC source E and, conversely, positive charges in the PIP body are conducted toward the edge of the PIP body 10, which is adjacent to the electrode 14 connected to the negative terminal of the DC source E. Such charges remain trapped upon termination of irradiation.

When the system is subjected only to image-wise radiation while the polarity of the DC source E is reversed, the PIP body 10 reacts as shown in FIG. 2. In FIG. 2 it is seen that only those areas of the PIP body subjected to the image-wise radiation undergo internal polarization under the force of the field produced by the reversed polarity of source E. The system has thus produced a latent electrostatic image (as represented schematically by the four negative charges on the right side of the PIP body 10 adjacent to mesh electrode 14 in FIG. 2) which is capable of being toned and transferred through the use of charged electroscopic particles (not shown). The discontinuous electrode 14 is removed for toning of the latent electrostatic PIP image.

Thus, it can be seen that the use of a discontinuous electrode such as the mesh electrode 14 in this embodiment provides a rather effective and efficient electrode for use with a PIP electrophotographic printing or copying system. Such a discontinuous electrode, being simple in structure and light in weight, is suitably adapted to be used as to top electrode in a PIP system as it is sufficiently transparent for the passage of light both for polarization and for image formation within the PIP material. It is further well suited to be used as the top electrode as it will provide easy removal for the purposes of toning the electrostatic image for reproduction of the image. An additional advantage of a discontinuous electrode, such as a screen mesh electrode, is that the mesh is flat and mechanically rugged and flexible. Thus, such mesh is easy to handle and conforms readily to the surface of the PIP material.

In commonly available copying machines, it has been found that the toning of a large solid area often results in decreased toner density; that is, deterioration of the image in areas furthest away from the edges. In other words, the middle portions of a large solid area which has been toned and transferred from such an image often appear less distinct than do the edge portions. The use of a discontinuous electrode (such as a mesh) electrically breaks up the large areas, thereby resulting in uniform development over the large solid areas of the latent image.

Thus, the invention as described herein provides a discontinuous electrode which is well suited for allowing the transmission of light and is removable for printing purposes as used in conjunction with a PIP electrostatic image forming system. It should be noted that although this invention has been described in connection with a planar system, it is well suited to be used in conjunction with a rotary drum system.

Although this invention has been described and illustrated exists between said electrodes and through the photoconductive body when an image is impressed on said photoconductive body through one of said electrodes, the improvement wherein said one of said electrodes is removable for electrostatic latent image toning purposes and comprises a discontinuous conductive electroformed nickel mesh having formed therein a plurality of openings constituting about 60% to about of the surface of said conductive mesh.

2. A discontinuous conductive electrode for use with a printing or copying system wherein a persistent electrostatic latent image is formed in a photoconductive body exhibiting persistent internal polarization when sandwiched between two conductive electrodes wherein an electric field exists between said electrodes and through the photoconductive body when an image is impressed on said photoconductive body through said discontinuous conductive electrode, said discontinuous conductive electrode being removable for elec,rostatic latent image toning purposes and comprising a substantially transparent, mechanically discontinuous conductive metal electroformed nickel mesh having formed therein a plurality of openings constituting between 60 to 90 percent of the surface of said conductive mesh.

F UNITED STATES PATENT OFFICE 5 9 CERTIFICATE OF CORRECTION Patent No. 3, 5 49, 115 Dat d Mar-ch 1", 1972 Inventofls) Maclin S. Hall It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- Column 2, line 20, "different" should be -dii''icu1t- .1 Column 5, Line 20, between "shown" and "schematica'l 1y", delete the word [schematic]. Column vline fil, "e1ec,rodes" should he --eLecLr=odes--; line 4-8, "e'leo,rostatio should be -elec.tr'ouLatic-; lines 19 and 50, delete [substantially transparent, mechanically].

Signed and sealed this 22nd day of August 191 2.

(SEAL) .Attest:

EDWAIHI) M.FLhITGHirLR,JR. ROBERT GOPTSGl-lALK Attosting Officer Commissioner of Patents P041050 aU -NI TED STATES PATENT 0mm v CERTIFICATE OF CORRECTION .l a tejt z t No 3,6 L9,*1 16 bated March 1A, 1972 M lin s Hall I j It is' ce r tififc l that; error appears in'theaboveidentifiedatenfi and th'at said Lette ps Patent are hem-gab czorarectzed as shown 'below:

r. CO1umn;2-, line 20, "different should be -dii"f'i cult- Column "3 line 20, between: shown" and. "schematical 1y",- delete the word [schematic] Column line 51., "e1ec ,r'0(ies" should "be --.ejLe 'cL'rodes-'; line 48, "e160,rostatic"should-bu el-e' c.tro:';tatic-fg Ilium; L9 and50, delete [substantially transparent, mechanically].

' Si-g and sealed this 22nd day of Au ust 1972-.

(S AN Attes-t;

EDWARD MLji LziflTfiHl llfiJRa ROBERT GOTT'SCHALK v Attastirig Office:- 1 Con'rzmissionear' of Pawnts 

1. In a printing or copying machine wherein a persistent electrostatic latent image is formed in a photoconductive body exhibiting persistent internal polarization when sandwiched between two conductive electrodes wherein an electric field exists between said electrodes and through the photoconductive body when an image is impressed on said photoconductive body through one of said electrodes, the improvement wherein said one of said electrodes is removable for electrostatic latent image toning purposes and comprises a discontinuous conductive electroformed nickel mesh having formed therein a plurality of openings constituting about 60% to about 90% of the surface of said conductive mesh.
 2. A discontinuous conductive electrode for use with a printing or copying system wherein a persistent electrostatic latent image is formed in a photoconductive body exhibiting persistent internal polarization when sandwiched between two conductive electrodes wherein an electric field exists between said electrodes and through the photoconductive body when an image is impressed on said photoconductive body through said discontinuous conductive electrode, said discontinuous conductive electrode being removable for electrostatic latent image toning purposes and comprising a substantially transparent, mechanically discontinuous conductive metal electroformed nickel mesh having formed therein a plurality of openings constituting between 60 to 90 percent of the surface of said conductive mesh. 